Voltage regulator circuit effective over predetermined input range

ABSTRACT

A CONSTANT VOLTAGE DEVICE WHICH IS ADAPTED TO PRODUCE A CONSTANT OUTPUT VOLTAGE WHEN VARIATIONS IN AN INPUT VOLTAGE ARE WITHIN A WORKING RANGE OF THE DEVICE. WHEN THE INPUT VOLTAGE OF THE DEVICE IS OUT OF SAID WORKING RANGE, THE OUTPUT VOLTAGE THEREOF IS REDUCED TO SUBSTANTIALLY ZERO.

United States Patent Inventor Shiro Suzuki Tokyo, Japan Appl. No. 810,760

Filed Mar. 26, 1969 Patented June 28, 1971 Assignee Meidensha Electric Mfg. Co., Ltd.

Tokyo, Japan Priority Mar. 29, 1968, Feb. 28 1969 Japan 43/20624 and 44/ 15750 VOLTAGE REGULATOR CIRCUIT EFFECTIVE OVER PREDETERMINED INPUT RANGE [50] Field of Search 317/31, 33;

Primary Examiner-J. D. Miller Assistant Examiner-A. D. Pellinen Attorney- Kelman and Herman 2 Chims4 Drawing Figs ABSTRACT: A constant voltage device which is adapted to U .5. Cl 323/22, produce a constant output voltage when variations in an input 323/38 voltage are within a working range of the device. When the Int. Cl G051 1/56, input voltage of the device is out of said working range, the

GOSf 1/58 output voltage thereof is reduced to substantially zero.

VOLTAGE REGULATOR CIRCUIT EFFECTIVE OVER PREDE'I'ERMINED INPUT RANGE The present invention relates to a device which produces a constant output voltage at all times when variations in an input voltage are within a working range. More particularly, the invention is concerned with a transistor operated constant voltage device of the series connection type.

In a known constant voltage device of the series connection type, a control transistor connected in series to a main circuit is controlled by a difference in voltage between a voltage proportional to the output voltage and a voltage reference, so that the output voltage is kept constant when the input voltage remains within a given range. However, if the input voltage exceeds the upper limit of the working range, then the device fails to function as a constant voltage device and the output voltage increases. This may cause damage to a circuit element connected to the constant voltage device as a load.

In order to prevent this phenomenon. it has hitherto been customary to adopt the system of preventing the firing of the control transistor when the fiow of output current (load current) is excessive as a result of a rise in the output voltage.

This system consists in connecting a resistor in series to the main circuit to prevent the firing of the control transistor by the voltage produced when an excess current flows through this resistor, whereby the output voltage is reduced to substantially zero. A disadvantage of this system lies in the fact that the control transistor is liable to be damaged or destroyed when the input voltage exceeds the maximum voltage level between the collector and emitter of the control transistor. To protect the control transistor against an excess voltage, a rapid acting fuse or an excess voltage relay has hitherto been used. However, the use of the former involves replacements each time the fuse melts while the use of the latter requires the use of an additional power source.

n the other hand, if the input voltage is reduced below the lower limit of the working range, then the device fails to function as a constant voltage device and the output voltage is reduced. For example, if the load is a computer, a reduction in the output voltage of the constant voltage device may result in a calculating error though the computer might operate as if it were normal.

The use of an undervoltage relay to prevent this error in operation also requires the use of an additional power source.

The present invention is based on the finding that better results can be obtained by reducing the output voltage to substantially zero when the input voltage is outside the working range. By this arrangement, it is possible not only to protect the control transistor against an excess voltage and an excess current but also to prevent an error in operation of the load because the load is not actuated. Moreover, the arrangement makes it possible to detect the abnormality as soon as it occurs.

Accordingly, an object of the present invention is to provide a constant voltage device whose output voltage is reduced to substantially zero when its input voltage is reduced below the lower limit of a working range of the device whereby an error in operation of the load can be prevented.

Another object of the invention is to provide a constant voltage device whose output voltage is reduced to substantially zero when its input voltage exceeds the upper limit of a working range of the device, whereby the device can be protected against an excess voltage and an excess current.

Still another object of the invention is to provide a constant voltage device which has a long service life and which is reliable in operation and economical to use, because the device is operated by purely electrical means without using a relay which requires an auxiliary power source and involves mechanical losses or fuses which require replacements whenever they melt.

Additional objects and advantages of the invention will become apparent from the description set forth hereunder when considered in conjunction with the accompanying drawings. in which:

FIG. I is a circuit diagram of a first embodiment of the device according to the present invention;

FIG. 2 is a diagrammatic representation of the characteristics of the device shown in FIG. 1;

FIG. 3 is a circuit diagram of a second device the present invention; and

FIG. 4 is a diagrammatic representation of the characteristics of the embodiment of FIG. 3.

The first embodiment of the device according to this invention which only provides for prevention of an error in operation of the load will now be explained with reference to FIG. I. In FIG. I. a control transistor Trl is connected in series to a main circuit. A resistor R1 is connected across the collector and the base of said control transistor Trl.

Voltage dividing resistors R3 and R4 for dividing an output voltage B0 are connected to the base of a detection transistor Tr2, and a Zener diode ZDI which provides a voltage reference and a resistor R2 are connected to the emitter of the detection transistor Tr2. The collector of the detection transistor Tr2 is connected to the base of the control transistor Trl, so as to thereby form a constant voltage circuit C of the series connection type.

The constant voltage circuit C operates in such a manner that the output voltage E0 is kept constant when variations in an input voltage El remain within a given range.

More specifically, a voltage proportional to the output voltage E0 is present across the voltage dividing resistors R3and R4 which voltage is applied to the base of the detection transistor Tr2 where it is compared with the Zener voltage (voltage reference) of the Zener diode ZDI to detect a variation in the output voltage E0. The control transistor Trl which is controlled by the detected voltage operates to restore the output voltage E0 to a predetermined constant value by eliminating the variation.

If the voltage of the Zener diode ZDI in the constant voltage circuit C is Ezl, the output voltage E will have a constant voltage Ec which is the sum of a voltage drop of R3 and a voltage drop (Ezl) of R4 when the input voltage Bi fluctuates within a given range, the voltage across the base and the emitter of the detection transistor Tr2 being disregarded. Therefore, the output voltage E0 can be expressed by the following equation:

according to The range of variations of the input voltage Ei which gives the output voltage E0 the constant value E0 obtained from this equation is the working range of the constant voltage device according to this invention as shown in FIG. 2.

The symbol S refers to a switching circuit which is a Schmidt circuit comprising transistors Tr3 and Tr4 and resistors R5 to R10 and utilizing the resistor R1 of the aforementioned constant voltage circuit C. A constant voltage element, for example a Zener diode ZD2, is connected in series to the base circuit of the first transistor Tr3 so as to prevent firing of the transistor Tr3 when the input voltage Ei is reduced below the lower limit of the working range.

More specifically, the Zener diode ZD2 is connected in series across the base of the first transistor Tr3 and the voltage dividing point where the input voltage Ei is divided by the resisters RS and R6 in the circuit S.

The collector of said transistor Tr3 is connected through the resistor R7 to one input terminal a and through the resistors R8 and R9 to the other input terminal b.

The connecting point of the resistors R8 and R9 is connected to the base of the second transistor Tr4 whose collector is connected to the base of the control transistor Trl of the constant voltage circuit C. The emitters of the first and second transistors TrJ and Tr4 are connected through the resistor R10 to the input terminal b.

The switching circuit S operates such that it reduces the output voltage E0 to substantially zero when the input voltage E1 is reduced below the lower limit El of the working range as shown in FIG 2 To enable the invention to be clearly understood, let us make the assumption set forth hereunder.

The assumptions may be inadequate'for accurately determining the circuit constant, but they are best suited for clearly explaining the operation of the switching circuit S.

The assumptions are:

l: The voltage Ebe3 across the base and the emitter of the first transistor Tr3 and the voltage Ebe4 across the base and the emitter of the second transistor Tr4 as well as the voltage Ece3 across the collector and the emitter of the transistor Tr3 and the voltage Ece4 across the collector and the emitter of and Tr4 are fired; and

ll. The Zener diode ZDZ has a very high nonworking impedance which is infinite, and the current flowing therethrough is disregarded. On the other hand, it has a very low working impedance which is substantially zero, and'the current flowing thereto is of very high value and variations in the value of this current do not cause variations in the Zener voltage.

On these assumptions, the conditions under which the first transistor Tr3 is fired and the second transistor Tr4 is deenergized will be expressed by the following relationship:

where Ez2 is the Zener voltage of the Zener diode ZD2, and Ee3 is the emitter voltage of the transistor Tr3.

The reason why the relation mentioned above holds will now be explained. Since the conditions under which the transistor Tr3 is fired and the transistor Tr4 is deenergized are sought, Tr3 is deenergized and Tr4 is fired now.

The input voltage E1 is divided by the resistors R5 and R6,

and also divided by the resistors R1 and R10 because the'voltage Ece4 across the collector and the emitter or the transistor Tr4 is disregarded in accordance with the assumption set forth in (II) above.

' Accordingly, the voltage ofthe resistor R6 can be expressed R6 R5 Rs' and the voltage of the resistor R10 or the emitter voltage Ee3 of the transistor Tr3 can be expressed by R 10 R 1 R 10' EZ If the voltage of the resistor R6 is higher than the sum of the emitter voltage Ee3 of the transistor Tr3 and the Zener voltage Ez2 of the Zener diode ZD2, a current flows across the base and the emitter of the first transistor Tr3 because of the voltage of the resistor R6, so that the resistance across the collector and the emitter of the first transistor Tr3 is lowered and consequently the transistor Tr3 is fired. In this case, firing of the first transistor Tr3 results in current not flowing across the base and the emitter of the second transistor Tr4, and the transistor Tr4 is deenergized.

The aforementioned relationship (2) can be rewritten as follows:

When the input voltage Ei is such that the relation (2) holds, the first transistor will be tired and the second transistor will be deenergized.

Thus, the input voltage El exerts no influences on the constant voltage operation of the constant voltage circuit C in which the control transistor Trl is controlled such that a voltage proportional to the output voltage which is taken out by the voltage dividing resistors R3 and R4 is compared with a voltage reference and the variation in the output voltage is removed. It will readily be seen that the conditions under which the first transistor Tr3 is deenergized and the second transistor Tr4 is fired can be expressed by the following relain other words. the first transistor Tr3 will be deenergized and the second transistor Tr4 will be fired when the input voltage Ei can be expressed by the following relation:

fl w. R 1 R 10 Also, when the current amplification degree of the Tr4 is [34 with respect to the base voltage Eb4 of the transistor Tr4, the

base current Ie4 of the transistor Tr4 can be expressed by the following relation:

And the current 19 flowing through the resistor R9 can be expressed by the following relation:

Thus, the current flowing to the resistors R7 and R8 will be le4+l9.

Therefore. the conditions set forth below has only to be satisfied if the aforementioned relation (3') is to hold:

If the values of the resistors R7 and R8 are selected such that the sum thereof is substantially equal to the value of the resistor R9 so that R1 R10,' the aforementioned relation (4) will be simplified as follows:

R7 RS What is important in the formula (4) or (4') is that the second transistor Tr4 can be fired by merely setting the circuit constant of the switching circuit S irrespective of. the input voltage Ei. Therefore, if the circuit constant is set such that the aforementioned relation (4') holds, the second transistor Tr4 will be fired when the input voltage satisfies the conditions (3'). Firing of transistor Tr4 results in the current flowing through the resistor R1 being shunted to the transistor Tr4, so that almost no current flows to the base of the control transistor Trl. This deenergizes the control transistor Trl, and the emitter voltage of the control transistor Trl or the output voltage E becomes substantially equal to the base voltage of Trl because the voltage Ebel across the base and the emitter of the transistor Trl is disregarded.

Thus, the output voltage will be reduced to substantially zero when R1 Rl0. If the marginal value iii of the input voltage that permits the relation (2) or (3) to hold is such that Ei Ei, then E0==0. The value Ei will thus be the lower or Eel= Ez' When Eo=0. the emitter voltage Ee3 of the first transistor Tr3 becomes R10 I RI ItlU because of the firing of the second transistor Tr4. Therefore.

the conditions for maintaining the first transistor Tr3 deenergized and the second transistor Tr4 fired or E ==O are R 10 Rl0 Rl-z-RlO Pl TRlO Thus, R1 R7. On the other hand. the conditions for l'f18lntaining the output voltage E0 at the level of a constant voltage value the minimum value of Ecel is assumed to be 3 volts on the safe side:

That is, the lower limit Ei, if the input voltage can be obtained from the relation (5'), so that E can also be expressed as follows:

Thus, the output voltage of the constant voltage device will never be less than Assuming that the base current begins to flow to the the first transistor Tr3 when Ei=Ei', the following relation can be obtained from the aforementioned relation (2):

' Ifkl and k2 are substituted for l I Rio Ro-l-RG Rl-l-RIO v respectively in the above relation, than the following relation can be obtained: E22

If the circuit constant is set such that the interrelation between kl, k2, E22 and E1" permits the aforementioned relation (7) to hold, the states of the first transistor Tr3 and the second transistor Tr4 will be reversed so that the former is fired and the latter is deenergized when the input voltage Ei reaches the lower limit E1 of the working range. When Ei Ei. EiEc. Thus. the output voltage can be kept Constant.

From the foregoing description, it will be appreciated that the present invention provides in one aspect a constant voltage device in which a control transistor connected in series to main circuit and having a resistor connected across the collector and the base thereof is controlled by a difference in voltage between a voltage proportional to the output voltage and a voltage reference so as -to maintain the output voltage con- LII stant when the input voltage remains within the working range.

The invention described with reference to the first embodiment thereof offers many advantages One of the advantages lies in the fact that since the output voltage is reduced to substantially zero when the input voltage is reduced below the lower limit of the working range, the load is not actuated so that any error in operation can be prevented. At the same time, the operator can readily and quickly detect the abnormal condition.

The second advantage is that the use of no contacts results in the device which responds quickly, is reliable and stable in performance, and has a long service life.

The third advantage is that the use of the control transistor of the constant voltage circuit as a switching transistor for reducing the output voltage to substantially zero permits the use of transistors of small capacity in the switching circuit for controlling switching of the control transistor from the state in which it is fired to the state in which it is deenergized or vice versa. Thus, the device of this invention is low in cost and therefore economical to use.

In the first embodiment of this invention having the construction and advantages described above, no means is provided for coping with that condition where the input voltage Ez' is increased above its upper limit Ei" or for coping with an increase in the output voltage E0 when the input voltage E0 is increased above the upperlimit Ei. In the second embodimentshown in FIG. 3, means is provided for coping with such state.

Turning to the second embodiment, the circuit shown in 'FIG. 3 is substantially similar to the circuit shown in FIG. 1 except for the fact that a second constant voltage element, for example a Zener diode ZD3, for deenergizing the first transistor Tr3 when the input terminal is increased above the upper limit of the working range is connected in parallel to the resistor R6 which is connected across the base and the emitter of the first transistor whereby a switching circuit 5 is formed.

It will be appreciated from the foregoing description that the second embodiment of the-constant voltage device according to the present invention performs a constant voltage operation when the input voltage Ei shows variations within the working range as shown in FIG. 4 and that it reduces the output voltage E0 to substantially zero when the input voltage is reduced below the lower limit Ei, provided that a third assumption (III) is made in addition to the assumptions (I) and (II), said third assumption being that the nonworking impedance of the Zener diode ZD3 is very high that the current flowing thereto is substantially zero, and the working impedance of the Zener diode is very low and substantially zero with respect to thecurrent flowing thereto, and the Zener 'voltage does not show any changes when there are variations in the current, and the Zener diode is not actuated when the input voltage is below the upper limit Ei" of the working range. In other words, the relations l to (7) described above hold in the second embodiment too if the assumptions (I), (II) and (III) are made.

The second embodiment of the constant voltage device illustrated in FIG. 3 operates such that when the input voltage exceeds the upper limit of the'working range (Ei Ei). the output voltage is reduced to substantially zero (Bo o).

The operation of this embodiment will be described hereinafter. In accordance with the assumption (III), the Zener diode ZD3 is not actuated when the input voltage is below the upper limit of the working range (Ei Ei). so that its impedance is infinitely high and the circuit operates as if the Zener diode Zd3 were not connected thereto. Thus. the second embodiment of the constant voltage device operates in the same manner as the first embodiment when the input voltage remains below the upper limit of the working range (Ei Eiah::).

In this state, the following relation holds:

where E53 is a terminal voltage of the Zener diode ZD3 and 52,3 is a Zener voltageof the Zener diode ZD3. Accordingly, if the value of 13:13 is determined such that Kl El Ez3 when Ei F!" then the Zener diode [D l\ ricllltlltfd u hen Ail-1 E 3, whereby the terminal voltage Ez3 (which is equal to MB) of the Zener diode ZD3 can be maintained equal to the Zener voltage E23.

On the other hand, the emitter voltage Ee3 of the'first transistor Tr3 when the latter is fired can be expressed by the A current does not flow to the baseof the first transistor Tr3 when the sum of its emitter voltage Ee3 and the Zener voltage Ez2 of the Zener diode ZD2 is higher than the Zener voltage E13 of the Zener diode ZD3 or when the following relation holds:

k3Ei+Ez2 Ez3 "(8) Thus, the first transistor Tr3 is deenergized when the input voltage satisfies equation (8). Deenergization of the first transistor Tr3 fires the second transistor Tr4, thereby deenergizing the control transistor Trl. Thus, the output voltage E is reduced to substantially zero in the same manner as when Ei Eiah.-.

If the upper limit Ei" of the working range at which Eo=0 when Ei=Ei" is substituted for Ei in relation (8). the value of Bi" can be expressed as follows:

From FIG. 4, it will be seen that E0=0 when Ei Ei.

From the foregoing description, it will be appreciated that the present invention provides in another aspect a constant voltage device of the type described as the first embodiment of the invention which further comprises a second constant voltage element connected in parallel to the resistor which is connected across the base and the emitter of the first transistor, whereby the first transistor is deenergized when the input voltage exceeds the upper limit ofthe working range.

In addition to the advantages recited with reference to the first embodiment, it will be evident that the invention offers the advantages described hereinafter.

The fourth advantage lies in the fact that the constant voltage device according to this invention provides protection not only to the load against excess voltages and excess current but also to the control transistor against destruction, because the device operates such that when the input voltage exceeds the upper limit of the working range the output voltage is reduced to substantially zero and the switching circuit maintains the output voltage substantially in the zero level while the input voltage and current are divided.

The fifth advantage is that the load is not actuated when the input voltage exceeds the upper limit of the working range, because the output voltage is reduced to substantially zero when the input voltage is out of the working range. The operator can readily detect the abnormal condition.

In the present invention, the resistor R10 is a resistor for positive feedback that forms a Schmidt circuit.

This resistor is used for improving the characteristics of the device in actual use, and its use is not essential from the theoretical view point. The value of this resistor is preferably minimized, for if the resistor R10 has a high value the residual voltage of the output voltage will be correspondingly high when the control transistor is deenergized. Thus, the residual predetermined upper limit and a predetermined lower limit,

voltage can be made substantially zero if R7 R10 and R1 Rl0. I

It is to be understood that present invention is not limited to the specific circuits described as the first and second embodiments of this invention, and that many changes and modifications may be made therein within departing from the spirit and scope of the invention.

It is also to be understood that the invention covers any circuit that may derive from the embodiments described so long as it reduces the output voltage to substantially zero when the input voltage is reduced below the lower limit of the working range or out of the working range.

I claim:

1. A voltage regulator circuit for providing a constant output voltage when the input voltage thereto varies between a the output of the voltage regulator falling to zero whenever the input voltage falls below said predetermined lower limit and rises above said upper limit, the circuit comprising:

a. a voltage-regulating control transistor having a base, an emitter and a collector, the emitter-collector circuit thereof being serially connected between a first input terminal and a first output terminal of said voltage regulator to maintain said output voltage constant, the second input terminal of said voltage. regulator being connected in common to the second output terminal thereof;

b. a first transistor, having a base, an emitter and a collector, the collector of said first transistor being connected to said first input terminal;

. a second transistor having a base, an emitter and a collector, the collector of said second transistor being connected to the base of said voltage-regulating control transistor;

1. the emitters of said first and second transistors being connected to said second input terminal through a first resistor,

. the base. of said first transistor being connected to a tap on a voltage divider comprising a second and a third resistor serially-connected between said first and second input terminals,

. the base of said second transistor being connected to the collector circuit of said first transistor, said first and second transistors and said first resistor forming a Schmidt trigger circuit with a fourth resistor which interconnects the base of said voltage-regulating control transistor to the collector circuit of said first transistor, and

d. a first constant-voltage device serially connected between the base of said first transistor and said voltage divider tap; 1. whereby, as the input voltage falls below said predetermined lower limit, said first constant-voltage device supplies a voltage to the base of said first transistor which turns said first transistor off, the action of said Schmidt trigger circuit then turning said second transistor on drawing base current from said voltageregulator control transistor and switching it off, thereby terminating operation of the regulator.

2. The voltage regulator as set forth in claim 1, further comprising:

a second constant-voltage device serially connected in the base circuit of said first transistor, across said second resistor, whereby, as the input voltage rises above said predetermined upper limit, said second constant-voltage device switches to a low impedance condition turning off said first transistor, the action of said Schmidt trigger circuit then turning on said second transistor and drawing base current from said voltage-regulating control transistor to turn it off thereby terminating operation of said regulator. 

